Method of coupling light into microresonators

ABSTRACT

A method of making a microresonator device includes the steps of providing at least a first substrate and providing a waveguide integrated on the substrate. The waveguide includes a core and a metal cladding layer on at least part of one boundary of the core. Another step is positioning a microresonator so that it is in an optically coupling relationship with the waveguide.

FIELD OF THE INVENTION

The present invention is directed generally to methods for couplinglight into optical devices based on microresonators coupled towaveguides.

BACKGROUND OF THE INVENTION

Dielectric cavity optical resonators have attracted increasing attentionin sensing applications, including biosensing. Typically theseresonators consist of either microspheres, or of planar-waveguide-baseddisk or ring cavities. The size of these types of resonators typicallyranges from approximately 20 microns to a few millimeters formicrospheres and from 5 microns to several hundreds of microns for ring-or disk-shaped resonators. Such small spheres and ring- or disk-shapedresonators are often referred to as microresonators.

In the most common configuration in microresonator-based sensors, amicroresonator is placed in close proximity to an optical waveguide suchas optical fiber whose geometry has been specifically tailored, forexample, tapered or etched to a size of 1-5 microns. The taperingmodifications to the waveguide result in there being a substantialoptical field outside the waveguide, and thus light can couple into themicroresonator and excite its eigenmodes. These eigenmodes may be ofvarious types, depending upon the resonant cavity geometry. Forspherical and disk cavities, the modes of interest for sensingapplications are usually the so-called “whispering gallery modes”(WGMs), which are traveling waves confined close to the surface of thecavity. Since the WGMs are confined near the surface, they arewell-suited to coupling with analytes on or near the sphere surface.FIG. 2 schematically illustrates the WGM 202 electric field distributionfor light propagating within a planar disk microresonator cavity 210.The field intensity, E, is schematically illustrated in FIG. 2 for theWGM 202 along the cross-section line A-A′.

For ring cavities based on single-mode waveguides, the modes are thoseof the single-transverse-mode channel waveguide, under the constraintthat the path traversed corresponds to an integral number ofwavelengths. Other cavity geometries, such as Fabry-Perot resonatorsusing single-mode waveguides with Bragg grating reflectors, or multimoderectangular cavities, have familiar standing-wave resonances as theireigenmodes.

When microresonators made with low loss materials and with high surfacereflectivity and quality are used, the loss of light confined in theresonant modes is very low, and extremely high quality factors, alsoknown as Q-factors, can be achieved, as high as 10⁹. Due to the highQ-factor, the light can circulate inside the resonator for a very longtime, thus leading to a very large field enhancement in the cavity mode,and a very long effective light propagation path. This makes suchdevices useful for non-linear optical and sensing applications. Insensing applications, the samples to be sensed are placed on or verynear the resonator's surface, where they interact with the evanescentportion of the resonant electric field available outside themicroresonator. Due to the enhanced field and the increased interactionlength between the light and samples, the microresonator-based opticalsensors feature high sensitivity and/or a low detection limit.

In the most-commonly-pursued configuration, in which a microsphereresonator is coupled to a tapered optical fiber, there are practicaldifficulties associated with realizing efficient and stable coupling.First, in order to make the optical field in the fiber core availableoutside the fiber's surface, the fiber must be tapered to a few micronsin diameter. This commonly results in a relatively long (a few cm) andfragile tapered region. Second, the relative position of the microsphereand the fiber taper must be held constant to within a few nanometers ifthe optical coupling and the Q-factor are to remain constant. This isdifficult with a free sphere and tapered fiber.

In another configuration commonly used to couple with microspheres, anangle polished fiber is put into contact with the microsphere. In thiscase the problems associated with the fragility of the fiber taper areovercome, but there are still significant difficulties in positioningthe microsphere properly on the fiber tip. Furthermore, light which isnot coupled into the microsphere, representing the so-called “throughport” signal, is not confined to a fiber and is therefore difficult tocollect and analyze.

There is a need for improved methods and structures for coupling awaveguide to a microresonator.

SUMMARY OF THE INVENTION

In a method of making a microresonator device, the steps includeproviding a substrate and providing a waveguide integrated on thesubstrate. The method further includes positioning a microresonator sothat the microresonator is in an optically coupling relationship withthe waveguide. The waveguide includes a core and a metal cladding layeron at least a portion of one boundary of the core.

The invention may be more completely understood by considering thedetailed description of various embodiments of the invention thatfollows in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C schematically illustrate embodiments of amicroresonator-based device.

FIG. 2 schematically illustrates a ray model, and the electric fielddistribution for light propagating within a whispering-gallery-mode fora planar disk microresonator.

FIG. 3 schematically illustrates an embodiment of a metal-clad waveguidecoupled to a spherical microresonator.

FIG. 4 schematically illustrates a lateral cross-section of theembodiment of FIG. 3 along the line 4-4.

FIG. 5 schematically illustrates an embodiment of a planar (disk orring) microresonator vertically coupled to a metal-clad waveguide.

FIG. 6 schematically illustrates a planar microresonator laterallycoupled to a metal-clad waveguide.

FIG. 7 schematically illustrates a planar microresonator laterallycoupled to a metal-clad waveguide, with the resonator modified forgreater surface sensitivity.

FIG. 8 schematically illustrates a planar microresonator laterallycoupled to a metal-clad waveguide, with the metal cladding positioned ina gap to strengthen the waveguide.

FIG. 9 schematically illustrates a planar microresonator laterallycoupled to a metal-clad waveguide, with metal and dielectric claddingpositioned in a gap to strengthen the waveguide.

FIG. 10 schematically illustrates a planar microresonator laterallycoupled to a metal-clad waveguide, with metal and dielectric claddingpositioned in a gap to strengthen the waveguide and with dielectriccladding over the top of the structure to add additional strength

FIG. 11 schematically illustrates three spherical microresonatorsvertically coupled to a metal-clad waveguide.

FIG. 12 schematically illustrates a planar microresonator laterallycoupled to a metal-clad waveguide, with metal positioned under thewaveguide core.

FIGS. 13 A, B and C show modeling results for an example slab waveguidestructure, showing the effect of the real and imaginary index of themetal cladding on the relative field enhancement and the waveguideattenuation for a wavelength of 633 nm.

FIGS. 14 A, B and C show modeling results for an example slab waveguidestructure, showing the effect of the real and imaginary index of themetal cladding on the relative field enhancement, and the waveguideattenuation for a wavelength of 1550 nm.

FIG. 15 illustrates a channel waveguide configuration for whichnumerical modeling was performed, the results of which are shown inFIGS. 16-18.

FIG. 16 shows a Beam Propagation Method (BPM) calculation of thedependence of the effective modal index on the thickness of the metallayer in FIG. 15, for a wavelength of 1550 nm, where the metal is gold.

FIG. 17 shows a BPM calculation of the relative field enhancement forthe metal-clad waveguide shown in FIG. 15, as a function of thethickness of the metal layer.

FIG. 18 shows a BPM calculation of the propagation loss for themetal-clad waveguide shown in FIG. 15, as a function of the imaginaryindex of the metal.

FIG. 19 is a graph showing experimentally obtained resonance plots foran embodiment of microresonator consisting of a microsphere coupled to ametal-clad waveguide, in accordance with principles of the invention.

While the invention may be modified in many ways, specifics have beenshown by way of example in the drawings and will be described in detail.It should be understood, however, that the intention is not to limit theinvention to the particular embodiments described. On the contrary, theintention is to cover all modifications, equivalents, and alternativesfollowing within the scope and spirit of the invention as defined by theclaims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is applicable to optical microresonator devices,in particular where an optical waveguide is coupled to an opticalmicroresonator. Devices based on the coupling of a waveguide to one ormore microresonators are particularly useful in the context of sensors,filters, telecommunications devices, and microlasers.

Metal clad waveguides are known, but have not previously been seen bythe inventors to be coupled to a microresonator. Historically, perhaps ametal clad waveguide has not been suggested for coupling to amicroresonator because metal claddings have a higher degree of opticalloss than other cladding structures. It has been discovered that byusing a metal clad waveguide with an optical microresonator, it ispossible to efficiently couple light from a metal-clad waveguide to amicroresonator with an acceptable amount of loss, and achieve many otherbenefits with some embodiments. More of these benefits will be describedherein.

The metal cladding layer is positioned in different locations indifferent embodiments. The waveguide core outer surfaces defineboundaries of the waveguide core. The metal cladding layer is on atleast a part of one boundary of the core. In some embodiments, the metalcladding is present on the waveguide core only along a section of coreproximate to the resonator. The metal cladding causes the shape of theoptical modes of the waveguide to be modified (see I. P. Kaminow et al,Metal-Clad Optical Waveguides: Analytical and Experimental Study,Applied Optics, Vol 13, page 396 (1974)), and can increase the strengthof the electric field that is present on the opposite boundary of thedielectric core from the metal. When the metal cladding is placed on theboundary of the core opposite the resonator, this enhancement of thefield strength allows the strength of coupling to the resonator to beincreased over what can be achieved with a conventional dielectriccladding. Alternatively, the gap between the waveguide and resonator canbe increased (to make the gap easier to fabricate) while maintainingadequate coupling.

Metals that achieve a significant shift of the mode to promote resonatorcoupling, but do not at the same time too strongly increase the loss ofthe waveguide are those that have a large value of the imaginary part oftheir refractive index “k” at the wavelength of operation. Examples ofmetals with large k values at visible and near infrared wavelengths arealuminum, gold, indium, silver, rhodium, sodium, iridium, magnesium,copper, rhenium, lead, molybdenum, platinum, zinc, nickel, strontium,niobium, tantalum, ytterbium, osmium, cobalt, iron, and vanadium.

This approach is primarily of benefit for the case of the transverseelectric “TE” polarized fundamental mode, where the electric field isparallel to the metal surface. In the case of transverse magnetic “TM”polarized modes (where the electric field is perpendicular to the metalsurface), the presence of the metal layer results in the fundamentalmode being the “plasmon” mode where the electric field is concentratednear the metal surface, and is thus not available for coupling to aresonator positioned at the opposite side of the core from the metal.Enhanced coupling between higher-order TM modes and resonators canpotentially be achieved, but generally the propagation loss of thehigher order TM modes is higher than TE modes, and thus the use of thisapproach in device designs is less attractive.

First, we will describe examples of microresonator-waveguide systemsgenerally and how they are used as sensors, in order to provide contextfor the discussion of the coupling systems. Then differentconfigurations and embodiments of a metal clad waveguide opticallycoupled to a microresonator will be described, along with advantages ofsuch systems. Finally, modeling and experimental results will besummarized.

Microcavity-Waveguide Systems

An example of a microcavity-waveguide device 100 is schematicallyillustrated in FIG. 1A. A light source 102 directs light along awaveguide 104 to a detector unit 106. The microresonator 110 isoptically coupled to the waveguide 104. Light 108 from the light source102 is launched into the waveguide 104 and propagates towards thedetector unit 106. The microresonator 110 evanescently couples some ofthe light 108 out of the waveguide 104, and the out-coupled light 112propagates within the microresonator 110. Both the coupling out of thewaveguide 104 and the intensity of light 112 in the micro-cavity aremaximized at one or more of the resonant frequencies of themicroresonator 110.

The light source 102 may be any suitable type of light source. Forincreased efficiency and sensitivity, it is advantageous that the lightsource produces light that is efficiently coupled into the waveguide104, for example the light source may be a laser such as a laser diode,or may be a light emitting diode. It is often advantageous that thelight source produce a narrow spectrum that is tunable, so thewavelength can be scanned to probe the resonances of the microcavity.The light source 102 generates light 108 at a desired wavelength, orwavelength range. For example, where the microresonator is used in asensor, the light source 102 generates light at a wavelength thatinteracts with the species being sensed. The species being sensed istypically located in proximity to the surface of the microresonator 110so that the light propagating in the resonator interacts with thespecies being sensed. The light source 102 may also comprise a lamp,along with suitable optics for coupling light from the lamp into thewaveguide 104.

Description of Microcavity-Waveguide Systems used as Sensors

There are several approaches to using the microcavity-waveguide systemas a sensor. The choice of approach is determined by a variety ofconsiderations, including the chemistry of the analyte to be detected,the time available for detection, the sample preparation technology,etc.

In one approach, detection is based on monitoring the intensity orwavelength of the light 108 that travels from the microresonator 110 tothe detector 106. This approach is based on the fact that when analytemolecules come in contact with the resonator surface and thus enter theevanescent field of the waveguide mode, they can alter the effectiverefractive index of the mode, and thus its equivalent path length. Thisresults in a shift of the resonant frequency of the resonator. The shiftin the resonant frequency can be detected either by scanning the inputwavelength and monitoring the resulting intensity profile, or by holdingthe input wavelength constant (but near a resonance) and detecting thechange in the intensity of light reaching the detector 106. Thisapproach has the benefit of being “label free”, in that no taggant needbe attached to the analyte in order to induce the signal change when theanalyte is proximal to the resonator.

In an alternate approach when the device 100 is used as a fluorosensor,the light propagating within the microresonator 110 is absorbed by afluorescent molecule, such as a fluorescent dye, that is in theproximity of the microresonator surface. This dye is associated with ananalyte to serve as a marker that indicates the presence of the analyte.In a more specific example, the surface of the microresonator may befunctionalized with antibodies specific to a desired analyte antigen.The analyte antigen molecules, conjugated with a fluorescent dye as partof the sample preparation step, are introduced to the sensor device 100.The antigen molecules bind to the antibody molecules on themicroresonator 110, thus holding the fluorescent dye moleculessufficiently close to the microresonator 110 that the light circulatingwithin microresonator 110 evanescently couples to the fluorescentmolecules. The absorbed light excites the fluorescent molecules and themolecules subsequently fluoresce at a wavelength different from theexcitation wavelength. Detection of the fluorescent light confirms thepresence of the analyte antigen.

In another example of a fluorosensor, the analyte antigen molecules arenot conjugated with a fluorescent dye, but are allowed to bind to theantibodies attached to the microresonator surface. More antibodies,conjugated to fluorescent molecules, are subsequently introduced to thesensor, and bind to the antigen. Again, the fluorescent molecules areexcited by an evanescent interaction with the light propagating withinthe microresonator 110, and detection of the subsequent fluorescence maybe used to determine the presence and abundance of the analyte antigen.

The light source 102 may direct light into a number of differentwaveguides, of which the waveguide 104 is one such example. According tothe invention, a metal clad waveguide is used to couple light into themicroresonator. In one embodiment, the waveguide 104 of FIGS. 1A-1C is ametal clad waveguide. More likely, the entire waveguide 104 is not ametal clad waveguide. Instead, a dielectric clad waveguide can couple toa metal clad waveguide near the point of optically coupling with themicroresonator. The remainder of the waveguide 104 may be any suitabletype of waveguide and may be, for example, a planar waveguide or achannel waveguide formed in or on a substrate, such as a waveguideformed in a silica substrate. The waveguide 104 may also be an opticalfiber.

The detector unit 106 includes a light detector, for example aphotodiode or photomultiplier, to detect light. The detector unit 106may also include a wavelength sensitive device that selects thewavelength of light reaching the light detector. The wavelengthselective device may be, for example, a filter, or a spectrometer. Thewavelength selective device may be tunable so as to permit the user toactively change the wavelength of light incident on the light detector.

The microresonator 110 may be positioned either in physical contactwith, or very close to, the waveguide 104 so that a portion of the light108 propagating along the waveguide 104 is evanescently coupled into themicroresonator 110.

Another type of microresonator device 150 is schematically illustratedin FIG. 1B. In this device 150, light 158 from the microresonator 110 iscoupled into a second waveguide 154, and propagates to the detector 106.

Another type of microresonator device 170 is schematically illustratedin FIG. 1C. In this device 170, a second detector 172 is positionedclose to the microresonator 110 to detect light from the microresonator110. The light detected by the second detector 172 does not pass to thesecond detector 172 via a waveguide, but rather via radiation modesthrough the surrounding medium (such as the liquid analyte beingsensed). The light from the microresonator 110 that is detected by thesecond detector 172 may be, for example, either scattered out of themicroresonator 110 or may be fluorescence arising from excitation of afluorescent species, near the surface of the microresonator, by lightcirculating within the microresonator 110. The second detector 172 maydetect all wavelengths of light from the microresonator 110 or, forexample, through the use of a wavelength selective element 174 placedbetween the second detector 172 and the microresonator 110, may detectlight that lies in a specific wavelength range. The wavelength selectiveelement 174 may, for example, be a filter that rejects light at theexcitation wavelength resonating within the microresonator 110 and thattransmits light at the fluorescent wavelength. The second detector 172may also be used with a configuration like that shown in FIG. 1B. Thesemicroresonator devices of FIGS. 1A-1C are described to provide contextfor the description of the microresonator-waveguide coupling structuresof the invention.

Configuration Examples for Coupling a Metal Clad Waveguide to aMicroresonator

There are many different examples of how a metal clad waveguide can becoupled to a microresonator resulting in a microresonator structure withan acceptable amount of optical loss and a simpler manufacturing processthan many other waveguide types.

One example of such a coupling is illustrated in FIGS. 3 and 4, where amicrosphere-waveguide structure 300 includes a microsphere resonator 304vertically coupled to a metal-clad optical waveguide 308. FIG. 4 is across-sectional view along line 4-4 in FIG. 3. Note that FIGS. 3 and 4are not to scale, since the waveguide width is typically much smallerthan the microsphere diameter, but the dimensions have been distortedhere to allow depiction of both the sphere and the waveguide structuraldetails. FIGS. 5-12 also are not to scale as the wave-guide width wouldtypically be much smaller than the resonator size in those examplesalso. Metal-clad waveguide 308 is constructed on a substrate 310, whichis a silicon wafer in this example. First, a dielectric cladding layer312 of, for example, silica, is grown on the silicon wafer 310. Then, ametallic layer 316 is formed, followed by the growth of the dielectriccore layer 320. The metallic layer 316 is chosen to serve as areflective interface to prevent the light from penetrating into thesubstrate. In one embodiment, the core layer is silica. The core 320 ispartially etched to form a core ridge 324. The dielectric core layer 320has a higher refractive index than the dielectric cladding layer 312.The core's outer surfaces define boundaries and the metallic layer 316is on at least a part of one boundary of the core. In the embodiment ofFIGS. 3-4, the metallic layer is on a portion of the bottom side of thecore 320 for the orientation shown in FIGS. 3-4.

In a variation of the above, the dielectric cladding layer may beomitted if the metal cladding will be present along the entire length ofthe waveguide. In that case, the metal would be deposited directly onthe silicon wafer 310.

In one embodiment, the metallic layer is gold, with a thickness of 150to 300 nanometers. In one embodiment, the width of the core ridge isabout 4.6 μm. The thickness of the core layer may vary from 1 micron to5 microns. Variations in the core layer thickness will achieve differentcoupling efficiencies, as will be demonstrated when simulation andexperimental results are described below.

FIG. 4 is a cross-section view of the embodiment of FIG. 3, taken alongthe line 4-4 of FIG. 3. FIG. 4 illustrates that the metal cladding layer316 is present only under the microresonator 304. Because the use of ametal cladding layer does increase the optical loss compared todielectric cladding layers, the metal cladding layer 316 is only presentin the vicinity of the microresonator in this embodiment. For example,in the embodiment of FIGS. 3-4, the length of the metal cladding layerin the direction of propagation is about 300 microns and the microsphere304 has a diameter of 300 microns. The metal cladding layer 316 iscentered with respect to the microsphere 304. In some embodiments, thewidth of the metal cladding layer (w in FIG. 4) ranges from 0.1 timesthe diameter of the microsphere to the length of the entire waveguide.

The channel waveguide structure 308 of FIGS. 3-4 is made monolithicallyon a planar substrate using semiconductor fabrication techniques. Thenthe microsphere is suspended above the surface of the channel, toproduce a hybrid configuration where the optical coupling between thesphere and the waveguide takes place in the vertical direction. Thisapproach preserves the high Q-factor of the glass microsphere, but doesnot solve the problem of how to precisely control the coupling betweenthe microsphere and the waveguide. (Approaches to controlling thecoupling between a microsphere and a channel waveguide have beenproposed in U.S. Published Patent Application No. 2005/0077513.)

Some of the above issues related to coupling a microresonator to awaveguide can be overcome by abandoning the hybrid system based on themicrosphere resonator, and using a fully-integrated system in which boththe coupling waveguide(s) and the resonant cavity are fabricated viaplanar processing. In this case, the problems of the accuracy andstability of the positions of the waveguide and resonator are solved.However, it is still difficult to control coupling due to the smalldistance (approximately 100 to 300 nm) that the waveguide field projectsoutside the waveguide channel. The reason for the small extent of this“evanescent” field is the high index contrast required to achieve lowloss in small resonators. As a result of the small extent of theevanescent field, good coupling between a waveguide and resonatornecessitates that a very narrow gap be fabricated between them,generally less than 1 micron wide. Accurate and repeatable fabricationof such narrow gaps is very difficult, especially since the waveguidelayers are typically several microns thick.

Examples will now be described where the entire microresonator-waveguidestructure can be manufactured monolithically. This is possible when theresonant cavity is a disk or ring, or other resonant cavity based on asingle mode channel or multimode planar waveguide rather than a sphere,so the microresonator and waveguide can be fabricated on the same planarsubstrate. This monolithic approach is typically realized in glass,polymer, or semiconductor waveguides, and provides excellent stabilityof coupling between the waveguides and the resonator. The etchingprocesses used to fabricate the microresonator, however, invariablyintroduce surface roughness, that results in a scattering loss therebydegrading the Q of the cavity. Cavities formed using this approachtypically have a Q-factor value of around a few thousand.

FIG. 5 illustrates an embodiment of a microresonator-waveguide couplingsystem 500 including a disk or ring microresonator 504 that isvertically coupled to a metal-clad optical waveguide 508. Waveguide 508is constructed on a substrate 510, which is a silicon wafer in thisexample. First, a dielectric cladding layer 512 of, for example, silica,is grown on the substrate 510. Next, a metal layer 516 is deposited andetched to provide a localized metal cladding region in the area wherecoupling to the resonator is desired. Next bus waveguide dielectric corelayer 520 is deposited and patterned. The waveguide core is then buriedunder more of the dielectric cladding material 512, and optionallyplanarized. Finally, the higher-index core layer for the resonant cavity504 is deposited and patterned. The core layer 520 has outer surfacesthat define boundaries of the core. The metallic layer 516 is on atleast a portion of the boundary of the core layer, in this case thebottom side of the core 520 for the orientation shown in FIG. 5. Themetallic layer 516 serves as a highly reflective cladding that preventsthe light from penetrating into the substrate, and forces the opticalmode in the bus waveguide up towards the microresonator to enhance thecoupling strength. In one embodiment, the core layer is also silica,with a dopant added to raise its refractive index. The bus waveguidecore layer 520, and the resonant cavity layer 504 have higher refractiveindices than the dielectric cladding layer 512.

The example of FIG. 5 has the microresonator vertically coupled to thewaveguide. It is also possible to have a lateral coupling relationship,as illustrated in FIG. 6 by microresonator-waveguide structure 600. Amicroresonator 604 formed as a ring or a disk is coupled to a metal cladwaveguide 608. The waveguide 608 is grown on a substrate 610 andincludes a dielectric cladding layer 612 and a bus waveguide core 620.Metal cladding 616 is positioned on the side 622 of the bus waveguidecore 620 that is opposite from the microresonator 604. The metalcladding 616 is also positioned on the top of the dielectric cladding612 that is immediately adjacent to the bus waveguide core 620. A gap624 is present between the microresonator 604 and the bus waveguide core620. The metal cladding 616 on the side 622 of the core serves to pushthe waveguide mode towards the resonator for enhanced coupling.

FIG. 7 illustrates a microresonator-waveguide coupling structure 700that is nearly identical to FIG. 6, except that an additional metalcladding portion 702 is present directly beneath the microresonator core704. In this configuration the optical mode in the resonator is pushedtowards the top of the resonator, resulting in stronger electric fieldat the top surface, thus increasing the surface sensitivity of themicroresonator. Note that this structure may result in larger opticalloss than in other embodiments because the metal cladding is in contactwith the resonator, but can be used in cases where achieving extremelyhigh Q is not critical, or perhaps not even desirable, as in the case offluorescence-based sensors. Identical reference numbers on FIGS. 6 and 7indicate identical elements.

FIG. 8 illustrates a microresonator-waveguide structure 800 with manysimilarities to the structure 600 of FIG. 6. A microresonator 804 formedas a ring or a disk is laterally coupled to a metal clad waveguide 808.The metal-clad waveguide 808 is grown on a substrate 810 and includes adielectric cladding layer 812 and a bus waveguide core 820. The metalcladding 816 is positioned on the side 822 of the bus waveguide core 820that is opposite from the microresonator 804. A gap 824 is presentbetween the microresonator 804 and the bus waveguide core 820. A featurenot present in FIG. 6 but illustrated in FIG. 8 is a reinforcementstructure 828 adjacent to the metal cladding layer 816. Thereinforcement structure 828 strengthens and supports the bus waveguidecore 820 and metal layer 816, thereby improving reliability. In oneexample, the reinforcement structure 828 is the same material as thewaveguide core 820.

FIG. 9 also illustrates a microresonator-waveguide structure 900 thatincludes a microresonator 904 formed as a ring or a disk that islaterally coupled to a metal clad waveguide 908. The waveguide 908 isfabricated on a substrate 910, and includes a dielectric cladding layer912 and a bus waveguide core 920. The metal cladding 916 is positionedon the side 922 of the bus waveguide core 920 that is opposite from themicroresonator 904, as well as on the top of the dielectric cladding 912that is immediately adjacent to the bus waveguide core 920. Thewaveguide 908 also includes a reinforcement structure 928 that issurrounded by the metal cladding layer 916 on three sides. Thereinforcement structure 928 strengthens and supports the bus waveguidecore 920, thereby improving reliability. In one example, thereinforcement structure 928 is a dielectric material which provides moremechanical strength than the metal layer. In addition, a section 930 ofthe same material as the waveguide core is present adjacent to the metalcladding 916 to provide additional mechanical stability and therebyimprove reliability.

FIG. 10 illustrates a microresonator-waveguide structure 1000 nearlyidentical to the structure 900 of FIG. 9, laterally coupling a disk orring microresonator 1004 and a waveguide 1008. However, a dielectricfill material 1028 is present not just adjacent to the metal claddinglayer 1016, but also over the entire top surface of the structure 1000and in the gap 1024 between a bus waveguide core 1020 and themicroresonator 1004. The waveguide 1008 is fabricated on a substrate1010 and includes a lower dielectric cladding layer 1012. The waveguide1008 includes a portion 1030 of waveguide core material for addedstability for the waveguide and metal structures. The dielectric fillstructure 1028 provides a relatively low cost technique to mechanicallyreinforce the microresonator, waveguide core 1020, and metal cladding1016 structures. Additionally, because the index of the fill material1028 is higher than that of air, the optical confinement in thewaveguide will be weakened, and the coupling between the resonator andwaveguide will be enhanced. In order to allow themicroresonator-waveguide structure 1000 to be used as a sensor, thedielectric fill material 1028 is permeable to liquid or gases so that ananalyte can move through the dielectric fill material 1008 to reach themicroresonator 1004. In an alternate approach, the fill material isdesigned to have the correct refractive index and thickness so that theresonator field will penetrate to its top surface and therefore beavailable for coupling to an analyte at the top surface of the fillmaterial 1028.

FIG. 11 illustrates a microresonator-waveguide structure 1100 with threemicro-sphere resonators 1102, 1103, 1104 that are coupled in a hybridfashion to a single channel waveguide 1108. The structure 1100 includesa substrate 1110, a dielectric cladding layer 1112 and a waveguide core1120. A single metal cladding layer section 1116 is positioned beneath aportion of the waveguide core 1120 and the three microresonators 1102,1103, 1104. At the boundaries of the metal layer, reflections areexpected that may in some cases interfere with the desired operation ofthe resonators, so it is desirable to group the three microresonatorsover one metal cladding layer, thereby eliminating the occurrence ofmultiple reflections. Of course, any number of resonators could becoupled to the same metal-clad core in this way.

FIG. 12 illustrates a microresonator-waveguide structure 1200 wheremetal cladding 1216 under a waveguide core 1220 causes the field to bepushed into the upper cladding 1228 on the resonator side, thusimproving the coupling. The structure 1200 includes a disk or ringmicroresonator 1204 laterally coupled to the waveguide 1208. A structurehaving metal cladding 1216 under the waveguide core is easier tofabricate than a structure with metal cladding on the side of thewaveguide core.

In FIG. 12, the dielectric fill material 1228 is present as the topsurface of the structure 1200 and in the gap 1224 between a buswaveguide core 1220 and the microresonator 1204. The waveguide 1208 isfabricated on a substrate 1210 and includes a lower dielectric claddinglayer 1212. The dielectric fill structure 1228 provides a relatively lowcost technique to mechanically reinforce the microresonator andwaveguide core 1220. Additionally, because the index of the fillmaterial 1228 is higher than that of air, the optical confinement in thewaveguide will be weakened, and the coupling between the resonator andwaveguide will be enhanced. In order to allow themicroresonator-waveguide structure 1200 to be used as a sensor, thedielectric fill material 1228 is permeable to liquid or gases so that ananalyte can move through the dielectric fill material 1228 to reach themicroresonator 1204. In an alternate approach, the fill material isdesigned to have the correct refractive index and thickness so that theresonator field will penetrate to its top surface and therefore beavailable for coupling to an analyte.

Microresonator Features

The microresonator typically has a diameter in the range from 10 μm tofive millimeters, but is more often in the range 10 μm-500 μm. Themicroresonator may be a ring resonator, a sphere resonator, a toroidalresonator, a disk resonator, a racetrack resonator, a rectangularresonator, a polygonal shape resonator, or a Fabry-Perot cavityresonator. A common diameter for a microsphere resonator is 300 μm,whereas resonators made by planar fabrication technologies can be mademuch smaller.

In some embodiments, the surface of the microresonator is modified forgreater surface sensitivity. For example, commonly-owned U.S.Publication No. 2005/0078731 describes porous microsphere resonatorsthat increase the amount of material that can be introduced to thesurface of a microresonator that has whispering gallery modes.

General Waveguide Features

The waveguide is often tapered to increase the intensity of the opticalfield outside the waveguide, thus increasing the amount of light thatcouples into the microresonator. In the case of an optical fiberwaveguide, the fiber may be heated and tapered by stretching, or may bechemically etched to a total thickness of about 1-5 μm. Likewise, with aplanar or channel waveguide, the waveguide core thickness or width maybe reduced at the region where the light is coupled to themicroresonator. In addition to the waveguide being reduced in size, thethickness of the cladding around the waveguide may also be reduced.

A stable relative position between the microresonator and a waveguidetaper can be difficult to achieve. Various approaches to establishingstable relative positions of the microresonator and the waveguide arediscussed in greater detail in commonly owned and co-pending U.S.Published Patent Application No. 2005/0077513, incorporated herein byreference.

Where a waveguide includes a core ridge, there are many options forpositioning a metal cladding layer relative to the core ridge. In oneembodiment, the metal coating is located beneath the core ridge. Inanother embodiment, the metal cladding layer is beneath at least a partof the core ridge. In yet another embodiment, the metal cladding layeris on at least a part of the side of the core ridge

In one embodiment, the waveguide core includes one or more of thefollowing materials: silica, silicon, silicon nitride, siliconoxynitride, titania, zirconia, Group III-V compound semiconductor, GroupII-VI compound semiconductor, and polymer. The silica material can havea dopant, such as germanium, phosphorous, or titanium in the silica.

The metal layer may be any metal which acts as a good reflector in theoptical frequency range in which the device is to be operated. That is,the imaginary part of the refractive index should be large, typicallygreater than 5. However, in order to further achieve low propagationloss, high magnitudes of the ratio of the imaginary part of therefractive index to the real part of the refractive index are desirable,generally such that k/n is greater than 5.

Table I provides values for optical constants n and k for severalmetals, measured at a wavelength of either 633 nm or 1550 nm, or both.Table I also shows the ratio k/n for some metals. In addition, Table Ishows the skin depth for each metal at 1550 nm. The skin depth iscalculated as follows:Skin depth=${{Skin}\quad{depth}} = \frac{0.08*1550\quad{mm}}{\left. \sqrt{}\left( {n*k} \right) \right.}$

It is important to be aware that measurements of optical constants formetals can vary depending on the particular material sample, themeasurement techniques, and other factors. As a result, the values inTable I may differ from optical constant values found from othersources. TABLE I Optical Constants for Some Metals of Potential Interestfor Waveguide Cladding Wavelength (nm) 1550 Skin 633 633 633 1550 15001500 Depth Metal n k k/n n k k/n (nm) Aluminum 1.4 7.7 5.5 1.44 16 11.126 Cobalt 4.5 5.8 1.29 24 Copper 0.21 3.7 18 0.61 8.3 13.6 55 Gold 0.173.2 19 0.56 11.5 20.5 49 Indium 2.3 11.3 4.91 24 Iridium 2.53 4.6 1.83.14 8.61 2.74 24 Iron 4.1 5.6 1.37 26 Lead 1.67 8.24 4.93 33 Magnesium0.48 3.7 7.7 2.04 8.6 4.22 30 Molybdenum 3.71 3.6 1 1.64 7.75 4.73 35Nickel 1.98 3.7 1.9 3.38 6.82 2.02 26 Niobium 2.52 2.5 1 2.3 6.6 2.87 32Osmium 3.88 1.7 0.4 2.00 5.95 2.98 36 Platinum 2.3 4.1 1.8 5.44 7.08 1.320 Rhenium 3.41 3.1 0.9 4.37 8.3 1.9 21 Rhodium 2.12 5.5 2.6 3.63 10.32.84 20 Silver 0.14 4.2 30 0.51 10.8 21.2 53 Sodium 0.03 2.6 87 0.45 920 62 Strontium 0.61 2.1 3.4 2 6.7 3.35 34 Tantalum 2.13 2.9 1.4 1.856.1 3.3 37 Vanadium 2 5.4 2.7 38 Ytterbium 0.78 3.3 4.2 1.63 6.1 3.74 39Zinc 3.25 4.3 1.3 1.47 6.97 4.74 39

Metals which are acceptable for the cladding layer include aluminum,gold, indium, silver, rhodium, sodium, iridium, magnesium, copper,rhenium, lead, molybdenum, platinum, zinc, nickel, strontium, niobium,tantalum, ytterbium, osmium, cobalt, iron, vanadium and alloys of theseelements. Gold, silver, aluminum, copper, and alloys thereof areparticularly useful as metal cladding layers at 1550 nm wavelength, dueto the lower loss of waveguides based on them.

These metals are selected to balance the field enhancement benefitsagainst the optical loss. The total optical loss to the microresonatordevice caused by the addition of the metal cladding will depend on howlong a region of the waveguide is coated with metal for coupling, whichdepends in turn on the size of the resonator. The coupling region willrange from 10 to 100 microns in length in many embodiments. Choosing thelongest value, 100 microns, to estimate a minimum for theattenuation/mm, it is reasonable to define “low loss” as less than 10dB/mm. Essentially any of the metals listed herein will work if loss of100 dB/mm is acceptable, corresponding to 1 dB in a 10 micron length. Tosome degree, the definition of low loss and acceptable loss isarbitrary, because the amount of loss that can be tolerated is a systemlevel issue that depends on how much power is available, how strong thesignal is, noise sources, and other factors.

Of course, the field enhancement and attenuation are wavelengthdependent, so the choice of metal will depend on the operatingwavelength. Based on calculations, the following trends emerge:

1) When k is >5 for the operating wavelength, most of the fieldenhancement is achieved. There is some benefit for k down to 2 or so,but the best performance required k>5.

2) When k/n is >5, the loss is predicted to be less than 10 dB/mm.

These principles can be applied to commonly used wavelengths ofinterest: 1550 nm and 633 nm. The list of metals that will provide goodfield enhancement at an operation wavelength of 1550 nm is as follows:aluminum, gold, indium, silver, rhodium, sodium, iridium, magnesium,copper, rhenium, lead, molybdenum, platinum, zinc, nickel, strontium,niobium, tantalum, ytterbium, osmium, cobalt, iron, and vanadium. Forloss <10 dB/mm, the list becomes: gold, silver, aluminum, sodium andcopper. Obviously, for chemistry reasons, sodium would not be easy touse. At an operating wavelength of 633 nm, the list of metals that willprovide good field enhancement is aluminum and rhodium. Of those, onlyaluminum also gives loss less than 10 dB/mm.

In some embodiments, the metal cladding layer is patterned. In someembodiments, the dielectric cladding layer is patterned. The thicknessrequirement on the metal layer is determined by the depth of theelectric field penetration into the metal, described by the skin depth(1/e decay length for the electric field). For optimal performance inmetal clad waveguides where the intent is to enhance coupling, the metalshould be thick enough that no significant electric field penetrates tothe side of the metal opposite the core. At an operating wavelength of1550 nm, the skin depths for typical metals are between 20 and 50 nm. Inorder to limit the optical mode to one side of the metal, a thicknessexceeding about two skin depths is desirable. Therefore, depending uponthe properties of the metal, a minimum thickness between 40 and 100 nmis appropriate. Note however that beyond exceeding the minimumthickness, control of the exact thickness of the metal is not required.Where the thickness of the metal is at least greater than twice the skindepth at the wavelength of operation, the result is that less than 2% ofthe optical power is on the wrong side of the metal, opposite the core.

A graded transition on a substrate is present in some embodimentsbetween a conventional waveguide and the metal clad waveguide. The metalclad waveguide in one example is an optical fiber taper with a metalcladding layer on one side of the taper.

Method of Assembling a Coupling of a Channel Waveguide to aMicroresonator

As mentioned previously, there are two primary categories of methods forassembling a coupling of a metal clad channel waveguide to amicroresonator: hybrid and monolithic. In both approaches, a firstsubstrate is provided, on which is fabricated a waveguide, where thewaveguide comprises a core and a metal cladding layer on at least aportion of one side. Then a microresonator is positioned so that themicroresonator is in an optically coupling relationship with thewaveguide. The two options are that the waveguide and resonator arefabricated as a monolithic integrated optical circuit, or the waveguideand resonator are fabricated separately and are then assembled as ahybrid optical circuit.

Advantages of Metal Clad Waveguides Over ARROW Structures

Non-fiber wave guides, such as integrated optical channel waveguides,have not found general use with microsphere resonators because of thedifficulty in making the optical field available outside of thewaveguide sufficiently large to obtain adequate coupling of the lightinto the microsphere. One example of channel waveguide structure thathas been used with microspheres is known as the Anti-Resonant ReflectingOptical Waveguide (ARROW). A more specific type of ARROW is a StriplinePedestal Anti-Resonant Reflecting Optical Waveguide (SPARROW). ARROWstructures optically isolate the waveguide core from the substrate witha high-reflectivity stack of alternating high- and low-index layers ofmaterials, such as Si and SiO₂, the thicknesses of which are defined asa quarter of the vertically directed guide wavelength. An early documentdescribing ARROW structures is U.S. Pat. No. 4,715,672 to Duguay et al.U.S. Pat. No. 6,657,731 to Tapalian et al describes the use of SPARROWwaveguides for coupling light into a microresonator in a chemicalsensor. A disadvantage of ARROW structures is that they include multiplelayers requiring thickness control during fabrication, and are thereforequite time consuming and complex to manufacture.

A metal-clad waveguide is very simple to manufacture compared to anARROW structure. Rather than growing multiple layers of alternating highand low index material, a simple single layer of metal may be usedaccording to the present invention. In addition, ARROW structuresrequire fairly accurate control of the reflector layer thickness, whilethe metal cladding layer of the present invention only need be thickerthan some minimum value without adversely affecting the performance ofthe structure.

It is very difficult to make a graded on-chip transition between aconventional waveguide and a ARROW. However, because of the smallerthickness of the metal layer, it is easier to make a graded on-chiptransition between a conventional waveguide and a metal clad waveguide.It is also easier to apply a metal coating on the side of a ridgewaveguide than it is to deposit a multilayer reflector there, in orderto enhance lateral coupling, as shown in FIGS. 6-10.

Modeling and Experimental Results

A series of modeling experiments were performed to determine the typicalcharacteristics of a metal clad waveguide. To this end, an analyticalapproach was used as well as commercially available Beam PropagationMethod (BPM) software to solve for effective modal indices, fieldamplitudes, and propagation losses for various metal clad waveguidestructures. Specifically, BeamPROP™ software was used as the BPMsoftware, which is available from RSoft Design Group, Inc. of Ossining,N.Y.

One goal of the modeling experiments was to determine the fraction ofthe waveguide electric field amplitude outside of the core on the sidewhere the resonator is positioned. It is believed that this value isimportant because it is related to the strength of coupling between thewaveguide mode and resonator modes. The Relative Field Increase isdefined as the ratio of this integrated amplitude for the specifiedconstruction to that for a reference construction.

The analytical approach was used to look for general trends by studyingthe characteristics of a metal-clad slab waveguide structure (seeKaminow et al, Metal-Clad Optical Waveguides Analytical and ExperimentalStudy, Applied Optics v 13, p 396 (1974)). The structure consisted ofthree layers: an infinitely thick metal layer; a 1 micron thick corelayer with a refractive index of 1.5; and an infinitely thick topdielectric cladding layer with a refractive index of 1.33. The referenceconstruction was identical except for a 1.45 index layer in place of themetal layer. The Relative Field Increase in the top cladding layer andthe calculated attenuation for wavelengths of 633 nm and 1550 nm areshown in FIGS. 13A-C and 14A-C.

FIG. 13A shows the Relative Field Increase at the top cladding layerplotted against the imaginary portion of the index of refraction of themetal, for six different values of the real portion of the index ofrefraction of the metal, for a wavelength of 633 nm.

FIGS. 13B and 13C show the Attenuation of TE0 polarized light in unitsof decibels per millimeter at a wavelength of 633 nm plotted against theimaginary portion of the index of refraction of the metal, for sixdifferent values of the real portion of the index of refraction of themetal. FIG. 13C shows the same data as FIG. 13B, but using a finer scalefor both axes.

FIG. 14A shows the Relative Field Increase at the top cladding layerplotted against the imaginary portion of the index of refraction of themetal, for six different values of the real portion of the index ofrefraction of the metal for a wavelength of 1550 nm. FIGS. 14B and 14Cshow the Attenuation of TE0 polarized light in units of decibels permillimeter at a wavelength of 1550 nm plotted against the imaginaryportion of the index of refraction of the metal, for six differentvalues of the real portion of the index of refraction of the metal. FIG.14C shows the same data as FIG. 14B, but using a finer scale for bothaxes.

Numerical modeling using beam propagation software was done to assessthe benefits of the metal cladding in realistic channel waveguideconfigurations (as opposed to the slab waveguide configurations forwhich the analyses are easily performed analytically). FIG. 15illustrates the channel waveguide configuration 1500 for which thenumerical modeling was performed.

The channel waveguide structure included a core 1502, and the core had awidth and height of 1.5 and 2 microns, respectively, and the index ofrefraction was 1.53. The starting refractive index and thickness for themetal layer 1504 was 0.56+i11.5 and 200 nm, respectively. From thisstarting point, the thickness and imaginary index of the metal wasvaried to look for dependencies. The core 1502 sits upon a silicasubstrate 1510, having an index of 1.45. An upper cladding 1508 is waterwith an index of refraction of 1.33. The Relative Field Increase wasmeasured in the right clad region 1506. For all cases, the wavelengthwas 1550 nm. The reference construction for the Relative Field Increasecalculation is the case with no metal. As a result, the index of whatwould be the metal region in the other models is 1.33 for the referenceconstruction.

With the metal clad structure of FIG. 15, the intention is to increasethe amount of electric field in the water clad region on the right sideof the core compared to the reference construction.

FIG. 16 graphs the dependence of the real and imaginary propagationconstant on gold layer thickness for the example channel waveguidestructure shown in FIG. 15. FIG. 17 illustrates the Relative FieldIncrease in the right clad region for the example channel waveguidestructure shown in FIG. 15, for varying gold layer thicknesses.

FIGS. 16 and 17 demonstrate that for the example structure of FIG. 15, arelatively thin layer of metal could be used. For example, the metallayer thickness in one embodiment is greater than 20 nm. In anotherembodiment, the metal layer thickness is greater than 40 nm. In yetanother embodiment, the thickness of the metal cladding is greater than50 nm. For another embodiment, the thickness of the metal cladding layeris greater than 80 nm. For metal cladding layer thicknesses of greaterthan 50 nm, the structure will avoid losses and leakage of the modethrough the metal cladding that may occur with thinner metal layers. Theactual thickness of metal required for a given structure will dependupon waveguide type, geometry and layer refractive indices, as well asthe metal used and the wavelength of operation.

FIG. 18 shows the propagation loss for the example structure shown inFIG. 15, while varying the metal imaginary index of refraction andholding the real part of the index of refraction to 0.56. Results fromFIG. 18 agree very well with the results from the analytical model shownin FIG. 14B, for the case where the real part of the index of refractionwas 0.5.

Experiments were carried out to determine the observed Q-factors ofmicroresonators coupled to a metal-clad waveguide. The observed qualityfactor is defined by Q_(obs)=2πν₀τ where ν₀ is the resonant frequencyand 1/τ is the cavity decay rate for a particular mode. It is determinedexperimentally using the relation$Q_{obs} = \frac{\lambda_{0}}{\Delta\quad\lambda_{0}}$where λ₀ and Δλ₀ are the resonance wavelength and ½ width respectively.Also, $\frac{1}{Q_{obs}} = {\frac{1}{Q_{int}} + \frac{1}{Q_{coup}}}$where Q_(int) is the intrinsic quality factor of the resonator andQ_(coup) is the contribution due to coupling to the waveguide.The fraction of light transmitted through the waveguide is measurableand equals$\frac{P_{trans}}{P_{inc}} = \left( {1 - \frac{2Q_{obs}}{Q_{coup}}} \right)^{2}$Notice that the maximum light intensity will be extracted from thewaveguide when Q_(coup)=Q_(int)Rearranging gives$Q_{coup} = \frac{2Q_{obs}}{1 \pm \sqrt{\frac{P_{trans}}{P_{inc}}}}$and $\frac{1}{Q_{int}} = {\frac{1}{Q_{obs}} - \frac{1}{Q_{coup}}}$

The example tested was that of FIGS. 3 and 4, where the microsphereresonator had a diameter of 300 microns and the waveguide included agold cladding layer with a thickness of 300 nm. The waveguide's corethickness was varied for different experiments. The width of the coreridge was 4.6 microns. The experimental arrangement was similar to thatshown in FIG. 1A. In each experiment, a gold-clad waveguide 108 wasplaced in contact with the microresonator 110 and both were immersed inwater. A tunable diode laser was used as light source 102 with the laserwavelength at 980 nm. The wavelength of the light emitted by the laserwas tuned by varying the voltage that drives a piezo-actuator inside thelaser. When the wavelength was on-resonance with one or more of thewhispering gallery modes 112 of the microresonator, the amount of laserpower coupled into the microresonator was increased, leading to a powerdrop at the detector 106.

FIG. 19 shows the detected signal at the detector as a function of lasertuning, where the core thickness was 2.0 micron thick. The observedQ-factor of the microresonator was estimated from these results to beapproximately 7×10⁶ and the efficiency was nearly 100%. The intrinsic Qfactor of the microresonator resonator was calculated to be 1.4×10⁷.

This result confirms that microresonators with high Q-factors can beeffectively coupled to a metal-clad waveguide.

In a different experiment, the core thickness was 2.5 microns thick. TheQ-factor of the microresonator system was approximately 3×10⁶ andcoupling efficiency was 78%. Another experiment was performed where thecore thickness was 3 microns thick. The Q-factor of the microresonatorsystem was approximately 1.5×10⁶ and coupling efficiency was nearly 60%.In another experiment, the core thickness was 4 microns thick. TheQ-factor of the microresonator system was approximately 5×10⁶ andcoupling efficiency was 30%.

During these experiments, it was found that the light attenuation orloss in the gold-clad waveguide coupled with the microsphere resonatorwas comparable to that of ARROW-structured waveguides. In addition, itwas found that the gold-clad waveguide transmits only TE polarizedlight.

As noted above, the present invention is applicable to microresonators,and is believed to be particularly useful where microresonators are usedin sensing applications. The present invention should not be consideredlimited to the particular examples described above, but rather should beunderstood to cover all aspects of the invention as fairly set out inthe attached claims. Various modifications, equivalent processes, aswell as numerous structures to which the present invention may beapplicable will be readily apparent to those of skill in the art towhich the present invention is directed upon review of the presentspecification. The claims are intended to cover such modifications anddevices.

The above specification provides a complete description of the structureand use of the invention. Since many of the embodiments of the inventioncan be made without parting from the spirit and scope of the invention,the invention resides in the claims.

1. A method of making a microresonator device comprising: providing atleast a first substrate; providing a waveguide integrated on thesubstrate, wherein the waveguide comprises a core and a metal claddinglayer on at least a portion of one boundary of the core; positioning amicroresonator so that the microresonator is in an optically couplingrelationship with the waveguide.
 2. The method of claim 1, wherein thewaveguide and resonator are fabricated as a monolithic integratedoptical circuit.
 3. The method of claim 1, wherein the waveguide andresonator are fabricated separately and are then assembled as a hybridoptical circuit.
 4. The method of claim 1, where the waveguide is atapered optical fiber.
 5. The method of claim 1, wherein the metalcladding layer is selected from a group consisting of aluminum, gold,indium, silver, rhodium, sodium, iridium, magnesium, copper, rhenium,lead, molybdenum, platinum, zinc, nickel, strontium, niobium, tantalum,ytterbium, osmium, cobalt, iron, vanadium, and alloys of these metals.6. The method of claim 1, wherein the metal cladding layer is selectedfrom a group consisting of gold, silver, aluminum, copper and alloysthereof.
 7. The method of claim 1, wherein a thickness of the metalcladding layer exceeds two times the skin depth of the metal at theoperating wavelength.
 8. A method of making a microresonator devicecomprising: providing at least a first substrate; providing a waveguideintegrated on the substrate, wherein the waveguide comprises a core anda metal cladding layer on at least a portion of one boundary of thecore; positioning a microresonator so that the microresonator is in anoptically coupling relationship with the waveguide; wherein thewaveguide and resonator are fabricated as a monolithic integratedoptical circuit; wherein a thickness of the metal cladding layer exceedstwo times the skin depth of the metal at the operating wavelength.